The present invention relates generally to fuel cells, and more particularly to transition metal carbides, nitrides and borides, and their oxygen containing analogs (e.g. oxycarbides) useful as water gas shift catalysts for use in producing hydrogen for chemical processing and petroleum refining, and reducing the carbon monoxide content of feeds to fuel cells.
The water gas shift (WGS) is an important reaction in the conversion of fossil fuels into hydrogen for use in processing chemicals and refining petroleum. An important emerging application is in the production of hydrogen for fuel cells. Fuel cells electrochemically convert fuel and oxidant directly into electricity. Because of their inherent high efficiencies and low emissions, fuel cells have gained significant interest from automobile manufacturers and their suppliers. Many manufacturers favor the use of proton exchange membrane (PEM) fuel cells operating with hydrogen from the processing of fossil fuels. The key fuel processing steps are (1) steam reforming and/or partial oxidation and (2) water gas shift.
Hydrocarbon steam reforming and partial oxidation are the principal reactions used to generate hydrogen. Hydrocarbon steam reforming is highly endothermic and usually requires temperatures in excess of 700xc2x0 C. to be effective (eqn. 1). Performance of the reformer is very sensitive to the composition of the fuel, consequently steam reforming is not considered to be very fuel flexible.
CnHm+nH2Oxe2x86x92nCO+(n+m/2)H2xe2x80x83xe2x80x83eqn. 1 
Hydrogen can also be extracted from hydrocarbons via partial oxidation reactions (see for example eqn. 2). Partial oxidation reactions are exothermic; however, because the reaction is not catalyzed, temperatures in excess of 1000xc2x0 C. are required to achieve the necessary rates. The product composition is regulated by controlling the amount of O2.
2CnHm+nO2xe2x86x922nCO+mH2xe2x80x83xe2x80x83eqn. 2 
In autothermal reforming, partial oxidation is coupled with steam reforming. The relative contribution of steam reforming versus partial oxidation can be controlled by choice of catalyst and operation conditions. For a given feed, the reaction temperature is lower than that for partial oxidation alone. Compared to steam reforming, autothermal reforming can be carried out in a smaller reactor volume, starts faster, and responds more quickly to control actions or changes in feed conditions.
The water gas shift reaction (eqn. 3) is well established for producing hydrogen and decreasing the CO content to less than 1%.
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83eqn. 3 
Carbon monoxide removal is critical because many catalysts are poisoned by CO. For example, the noble metal electrocatalysts in PEM fuel cells are susceptible to poisoning by as little as 10-100 ppm CO. The poisoning problem is exacerbated by the operating constraints imposed by commercial membrane materials. Present PEM fuel cells must be operated under conditions which avoid drying out the membrane. This essentially excludes operating the fuel cell at the higher temperatures where Pt oxidizes CO. The water gas shift reaction is typically carried out in two stages using Fexe2x80x94Cr catalysts in the high temperature stage and Cuxe2x80x94Znxe2x80x94Al catalyst in the low temperature stage.
Presently employed catalysts lack sufficient activity and durability for many portable and automotive applications. Furthermore, presently available catalysts are very sensitive to sulfur compounds, a common contaminant in modern transportation fuels.
Therefore, there exists a pressing need for water gas shift catalysts that are highly active, durable, and sulfur tolerant. These materials would be especially well suited for use in conjunction with PEM fuel cells for automotive applications.
U.S. Pat. No. 3,666,682 to Muenger, the entire specification of which is incorporated herein by reference, discloses a water gas shift conversion process in which a feed gas mixture is subjected to successive contacts with catalyst and the temperature of the reacting gases contacting the shift conversion catalyst is controlled by indirect concurrent heat exchange with the feed gas mixture.
U.S. Pat. No. 3,974,096 to Segura et al., the entire specification of which is incorporated herein by reference, discloses that hydrogen is produced by reacting carbon monoxide with steam at a temperature of at least 200xc2x0 F. in the presence of a supported catalyst containing: (1) at least one alkali metal compound derived from an acid having an ionization constant below 1xc3x9710xe2x88x923, (2) a metallic hydrogenation-dehydrogenation material, and (3) a halogen moiety. The ratio of metal component to alkali metal compound, each calculated on the basis of the oxide thereof, ranges from 0.0001 to about 10 parts by weight per part by weight of the alkali metal compound. The halide constituent is present in amounts in excess of about 0.01 weight %, based on total catalyst. A preferred catalyst composition comprises potassium carbonate, a mixture of cobalt and molybdenum oxides and combined chlorine contained on an alumina support.
U.S. Pat. No. 4,172,808 to Bxc3x6hm et al., the entire specification of which is incorporated herein by reference, discloses a process for the production of a tungsten carbide catalyst by carburization of tungsten oxides, comprises, directing a mixture of carbon monoxide and carbon dioxide over tungsten oxide while heating it in a heated reactor at a heating rate and gas flow rate such that the reduction of the tungsten oxide occurs more slowly than the diffusion of the carbon into the tungsten and into tungsten carbide which is formed during the reaction with the diffusion being faster than the separation of carbon from the gaseous phase according to the rate of adjustment of the Boudouard equilibrium. The carbon monoxide is charged at a rate of 560 l/h and the carbon dioxide is charged at a rate of 40 l/h and, after a reactor containing the sample of tungstic acid is positioned in a closed reactor, the reactor is flushed with the gases for around ten minutes and then placed into a muffle furnace. The reactor is heated to a temperature of 670xc2x0 C. in the furnace and the temperature is then reduced to a reaction temperature of 620xc2x0 C. First, all of the water is eliminated, and then there is a reduction of the tungsten oxides and a diffusion of the carbon into tungsten or into tungsten carbide which is formed. The reduction of the tungsten oxides occurs more slowly than the diffusion of the carbon, but faster than the deposition of the carbon from the gaseous phase.
U.S. Pat. No. 4,219,445 to Finch, the entire specification of which is incorporated herein by reference, discloses a process of preparing methane-containing gas comprising contacting carbon monoxide and hydrogen in the presence of a catalyst containing tungsten carbide. Various tungsten carbide-containing alumina gel catalysts are also disclosed.
U.S. Pat. No. 4,271,041 to Boudart et al., the entire specification of which is incorporated herein by reference, discloses a high specific surface area molybdenum oxycarbide catalyst. They are prepared by the vapor condensation of molybdenum hexacarbonyl and catalyze the reaction of hydrogen and carbon monoxide to form hydrocarbons. Carburization of the molybdenum oxycarbides increases their activity in the carbon monoxide-hydrogen reaction.
U.S. Pat. No. 4,325,842 to Slaugh et al., the entire specification of which is incorporated herein by reference, discloses a process for preparing a supported molybdenum carbide composition which comprises impregnating a porous support with a solution of hexamolybdenum dodecachloride, drying the impregnated support and then heating in a carbiding atmosphere at a temperature of about 650xc2x0-750xc2x0 C.
U.S. Pat. No. 4,325,843 to Slaugh et al., the entire specification of which is incorporated herein by reference, discloses a process for preparing a supported tungsten carbide composition which comprises first forming a supported tungsten oxide composition, converting the oxide to the nitride by heating in an ammonia atmosphere, and then converting the nitride to the carbide by heating in a carbiding atmosphere.
U.S. Pat. No. 4,789,534 to Laine, the entire specification of which is incorporated herein by reference, discloses transition metal carbides in which the carbon is in excess and is covalently bound to the metal are produced by pyrolyzing transition metal amides that have two or more metal atoms, such as hexakis (dimethylamido) ditungsten or dimolybdenum.
U.S. Pat. No. 4,808,563 to Velenyi, the entire specification of which is incorporated herein by reference, discloses a catalyst which comprises a molybdenum-tungsten-containing complex represented by the formula MoaWbMcAdOe, wherein M is selected from the group consisting of one or more metals selected from any of Groups IB, IIB, IVB, VB or VIII of the Periodic Table and/or one or more of Y, Cr, Mn, Re, B, In, Ge, Sn, Pb, Th or U, or a mixture of two or more of the metals in said group; A is at least one metal selected from the group consisting of alkali metals, alkaline earth metals, Lanthanide series metals, La, T1, or a mixture or two or more of the metals in said group; a is a number in the range of from about 1 to about 200; b is a number in the range of from about 1 to about 200; with the proviso that either Mo or W is in excess of the other, the ratio of a:b being about 4:1 or greater, or about 1:4 or less; c is a number such that the ratio of c:(a+b) is in the range of from 0:100 to about 10:100; d is a number such that the ratio of d:(a+b) is in the range of from 0:100 to about 75:100; and e is the number of oxygens needed to fulfill the valence requirements of the other elements. A process for converting gaseous reactants comprising methane and oxygen to higher order hydrocarbons using the foregoing catalyst is also disclosed.
U.S. Pat. No. 4,812,434 to Pohlmann et al., the entire specification of which is incorporated herein by reference, discloses an exhaust gas catalyst, wherein it consists of about 50 to about 95% by weight of silicon carbide and about 5 to about 50% by weight of an alloy of silicon with one or more metals of the group copper, iron, cobalt, nickel, chromium, vanadium, molybdenum, manganese, zinc, silver, platinum, palladium or other catalytically-active metals, the catalytically-active surface of which has optionally been activated by oxidation and/or chemical after-treatment.
U.S. Pat. No. 4,851,206 to Boudart et al., the entire specification of which is incorporated herein by reference, discloses methods and compositions produced thereby concerning the preparation and use of high specific surface area carbides and nitrides. The carbides and nitrides can be obtained by thermal reduction of oxides in the presence of a source of carbon and nitrogen respectively, with relatively slow progressive temperature increases prior to completion of the reaction, followed by quenching. Novel metastable carbides can be obtained by carburization of nitrides having high surface area, which nitrides can be prepared by the above-described process.
U.S. Pat. No. 5,039,503 to Sauvion et al., the entire specification of which is incorporated herein by reference, discloses that carbon monoxide is reacted with water vapor and converted into hydrogen and carbon dioxide, in the presence of a thio-resistant catalyst which comprises an active phase deposited onto a support, said active phase comprising molybdenum, vanadium or tungsten, and a cobalt and/or nickel promoter therefor, and said support comprising cerium oxide or zirconium oxide. The reaction mixture includes carbon monoxide, hydrogen, water and compounds of sulfur, wherefrom hydrogen is selectively produced in increased amounts.
U.S. Pat. No. 5,321,161 to Vreugdenhil et al., the entire specification of which is incorporated herein by reference, discloses that nitrides can be hydrogenated to amines by heating the nitrile in the presence of hydrogen and a tungsten carbide catalyst, such as are formed by the calcination of a tungsten salt with an acyclic compound containing-nitrogen-hydrogen bonding.
U.S. Pat. No. 5,444,173 to Oyama et al., the entire specification of which is incorporated herein by reference, discloses bimetallic oxynitrides and nitrides which have catalytic properties comprise two transition metals selected from Groups IIIB to VIII of the Periodic Table of the Elements. Preferably, one metal is either molybdenum or tungsten. The other can be tungsten or molybdenum, respectively, or another transition metal, such as vanadium, niobium, chromium, manganese, cobalt, or nickel. They have a face centered cubic (fcc) arrangement of the metal atoms and have a surface area of no less than about 40 m2/gm.
U.S. Pat. No. 5,468,370 to Ledoux et al., the entire specification of which is incorporated herein by reference, discloses a catalyst for chemical and petrochemical reactions and a process for its production. The catalyst comprises an oxide of one of the transition metals, rare earth elements, or actinide elements, e.g., molybdenum, having on its surface carbides and oxycarbides, the core being the metal or the metal oxide. In the process for catalyst production, the reaction gas mixture containing carbon products is passed onto the oxide, leading to a progressive carburization of the surface of the oxide and to a progressive increase in the efficiency of the catalyst.
U.S. Pat. No. 5,821,190 to Kurabayashi et al., the entire specification of which is incorporated herein by reference, discloses a catalyst and method for purifying exhaust gases, having superior performance of NOx purification to exhaust gases containing oxygen and nitrogen oxides, particularly superior performance of NOx elimination to exhaust gases from lean-burn engines with excess oxygen, and a wider effective temperature range of NOx elimination, and also superior heat resistance at high temperature. The catalyst for purifying exhaust gases comprises, as indispensable contents, iridium and alkaline metal loaded on a carrier which is at least one selected from metal carbide and metal nitride, or these and at least one element selected from the group consisting of alkaline earth metal elements and rare earth metal elements.
In accordance with one embodiment of the present invention, a catalyst for catalyzing the water gas shift reaction is provided, comprising the formula:
xe2x80x83M1AM2BZCOD 
wherein M1 is a transition metal;
M2 is a transition metal;
A is an integer;
B is 0 or an integer greater than 0;
Z is selected from the group consisting of carbon, nitrogen, boron, and combinations thereof;
C is an integer;
O is oxygen; and
D is 0 or an integer greater than 0.
In accordance with another embodiment of the present invention, a catalyst for catalyzing the water gas shift reaction is provided, comprising the formula:
M1AM2BZCOD 
wherein M1 is selected from the group consisting of molybdenum, tungsten, and combinations thereof;
M2 is selected from the group consisting of iron, nickel, copper, cobalt, and combinations thereof;
A is an integer;
B is 0 or an integer greater than 0;
Z is selected from the group consisting of carbon, nitrogen, boron, and combinations thereof;
C is an integer;
O is oxygen; and
D is 0 or an integer greater than 0.
In accordance with another embodiment of the present invention, a method is provided for catalyzing the water gas shift reaction in which carbon monoxide levels in a hydrogen-containing stream are reduced, comprising:
providing a catalyst having the formula:
M1AM2BZCOD 
wherein M1 is a transition metal;
M2 is a transition metal;
A is an integer;
B is 0 or an integer greater than 0;
Z is selected from the group consisting of carbon, nitrogen, boron, and combinations thereof;
C is an integer;
O is oxygen;
D is 0 or an integer greater than 0; and
exposing the hydrogen-containing stream to the catalyst for a sufficient period of time to reduce the carbon monoxide levels in the hydrogen-containing stream.
In accordance with another embodiment of the present invention, a method is provided for catalyzing the water gas shift reaction in which carbon monoxide levels in a hydrogen-containing stream are reduced, comprising:
providing a catalyst having the formula:
M1AM2BZCOD 
wherein M1 is selected from the group consisting of molybdenum, tungsten, and combinations thereof;
M2 is selected from the group consisting of iron, nickel, copper, cobalt, and combinations thereof;
A is an integer;
B is 0 or an integer greater than 0;
Z is selected from the group consisting of carbon, nitrogen, boron, and combinations thereof;
C is an integer;
O is oxygen;
D is 0 or an integer greater than 0; and
exposing the hydrogen-containing stream to the catalyst for a sufficient period of time to reduce the carbon monoxide levels in the hydrogen-containing stream.
A more complete appreciation of the various embodiments and aspects of the present invention and the scope thereof can be obtained from a study of the accompanying drawings, which are briefly summarized below, the following detailed description of the invention, and the appended claims.